Liquid crystals are useful for electronic displays because light travelling through a thin film of liquid crystal is affected by the birefringence of the film, which can be controlled by the application of a voltage across the film. Liquid crystal displays are desirable because the transmission or reflection of light from an external source, including ambient light, can be controlled with much less power than is required for luminescent materials used in other displays. Liquid crystal displays are now commonly used in such applications as digital watches, calculators, portable computers, and many other types of electronic equipment where the need exists for long-lived operation, with very low voltage and low power consumption. In particular, portable computer displays benefit from liquid crystal displays where display power utilization must be minimized to permit the battery to operate for as long a period of time as possible before recharging, while allowing the majority of the battery utilization to be directed toward computational efforts.
Glass has typically been used in visual displays which utilize liquid crystal materials. However, the use of glass has a number of disadvantages. For reasons of cost, alkali metal silicate glasses are used, and these must be coated with SiO in order to prevent the migration of the alkali metal. Additionally, the ultimate shapes are somewhat limited, in that only certain shapes can be economically processed. Chipping of glass plates is also an inherent problem.
There have been attempts to use transparent plastics as a substitute for glass. Transparent polymers can be inexpensively molded, as opposed to requiring more expensive grinding processes, but they are typically anisotropic, causing light to be refracted through them along several paths. This birefringence leads to the distortion of light waves passing through optical products made from such plastics, and detrimentally affecting their performance. U.S. Pat. No. 4,228,574 describes a liquid crystal display using plastic. However, as a rule, plastics are neither particularly isotropically, nor particularly mechanically or chemically stable, nor inexpensive. However, as shown in the '574 patent, they are well suited for automated production. It is evident, that the technology of glass displays cannot be simply transferred to plastics. Although a plastic can be found which closely matches, or is even superior to, the properties of the glass for each process step of the glass technology, no cell made of plastic has yet been described which would be equally as good as the glass cell, simply because all of the properties must be found by one plastic.
It has been proposed to use polyesters in place of glass. As a film, polyester is highly birefringent, and thus cannot be employed in the customary components using polarizers. Cellulose butyrate is isotropic, but is unsatisfactory in respect of its mechanical, especially thermal, and chemical properties, and has undesirable light scattering properties. Polycarbonate has also been proposed, but it is highly birefringent.
This invention generally pertains to optically isotropic devices or components, and more particularly to optically isotropic devices or components made of an optically isotropic polymeric material which is a blend of at least two completely miscible polymers. By the term, "optically isotropic", it is intended to refer to the properties in certain materials wherein their optical properties are the same in all directions, such properties including the index of refraction and light absorption.
Very few materials are optically isotropic. Few, if any molded organic polymeric materials are optically isotropic. Such transparent or partially transparent polymers as polyethylene, Lucite.RTM., trade mark of the E.I. duPont de Nemours Company, polymethylmethacrylate (PMMA), etc., are not optically isotropic. This may be seen by making a relatively thin layered sample of the polymeric material and then determining its birefringence and absorption of polarized light. Birefiingence of the sample is determined by finding the indices of refraction of the sample for polarized light in one direction and that for polarized light in a direction perpendicular to the first direction. The difference in the two indices of refraction is the birefringence of the sample material.
Even when a polymeric material has zero birefringence in its bulk state, the processing of such a material into a device, such as by extrusion or injection molding places stresses on the material in the direction of flow. Such mechanical stresses induce orientation of the polymer molecules, which almost always results in flow-induced birefringence.
Thus it can be seen that what is still lacking in the prior art is a material which can cycle between low and high temperature without associated delamination, the refractive index of which can be adjusted to fit the needs of the application, and the birefringence of which can be tailored.